StumbleUpon is a toolbar extension for your browser. Its primary button, labeled "Stumble!," takes you to a site in any of the categories you've told the service you're interested in but that you've probably not been to before. If you like the new site, you press a thumbs-up button. If you don't, thumbs-down. Every time you vote, the system recalibrates its site selections for you using social filtering; it attempts to find sites for you that it thinks you'll like because other users with similar preferences also like them.

The site is also a social network, with Web site preferences acting as the major connective tissue. The system recommends friends with similar browsing preferences; it can be enlightening to see the sites they've voted for.